Detection of superhumps in XTE J1118+480 approaching quiescence

C. Zurita; J. Casares; T. Shahbaz; R. M. Wagner; C. B. Foltz; P. Rodriguez-Gil; R. I. Hynes; P. A. Charles; E. Ryan; G. Schwarz; S. G. Starrfield

doi:10.1046/j.1365-8711.2002.05450.x

arXiv · astro-ph/0202438

Detection of superhumps in XTE J1118+480 approaching quiescence

C. Zurita·J. Casares·T. Shahbaz·R. M. Wagner·C. B. Foltz·P. Rodriguez-Gil·R. I. Hynes·P. A. Charles·E. Ryan·G. Schwarz·S. G. Starrfield

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Abstract

We present the results of our monitoring of the halo black-hole soft X-ray transient (SXT) XTE J1118+480 during its decline to quiescence. The system has decayed 0.5 mags from December 2000 to its present near quiescent level at R=18.65 (June 2001). The ellipsoidal lightcurve is distorted by an additional modulation that we interpret as a superhump of P_sh=0.17049(1) d i.e. 0.3% longer than the orbital period. This implies a disc precession period P_prec= 52 d. After correcting the average phase-folded light curve for veiling, the amplitude difference between the minima suggests that the binary inclination angle lies in the range i=71-82 deg. However, we urge caution in the interpretation of these values because of residual systematic contamination of the ellipsoidal lightcurve by the complex form of the superhump modulation. The orbital--mean H-alpha profiles exhibit clear velocity variations with ~500 km/s amplitude. We interpret this as the first spectroscopic evidence of an eccentric precessing disc.

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C. Zurita, J. Casares, T. Shahbaz, R. M. Wagner, C. B. Foltz, P. Rodriguez-Gil, R. I. Hynes, P. A. Charles, E. Ryan, G. Schwarz, S. G. Starrfield. 2002-02-23. Detection of superhumps in XTE J1118+480 approaching quiescence. https://doi.org/10.1046/j.1365-8711.2002.05450.x

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